Functional Magnetic Resonance Imaging of Olfactory Stimulus Processing D. Hommer Unlike visual or tactile perception, the functional anatomy of odor perception in humans has received very limited attention. This is unfortunate because the brain regions involved in odor perception appear to overlap with the brain regions involved in motivation and emotion. Since, in alcoholics, states of craving for alcohol can be induced by the odor of alcoholic beverages and these states involve both motivational and emotional components, we felt, as a prelude to studies of the functional neuroanatomy of alcohol craving, it would be important to develop techniques to examine brain changes associated with olfactory perception. Normal volunteers were exposed to various odorants using a continuous airflow system, while lying in a standard 1.5 Tesla MRI scanner. A pulse sequence developed at the In Vivo NMR Center was used to image blood volume under controlled conditions. This type of scan does not require injection of contrast or labeling agents. For each subject, 24 functional image data sets were acquired, in addition to anatomical images. Each trial consisted of one "rest state" scan (odorless air), followed by one "activated state" scan (odorant). Three types of odors were used eight times each: pleasant, unpleasant, and alcohol beverage odors. Each scan lasted 20 seconds, during which a three dimensional volume-image of 64 by 64 by 10 voxels was acquired. Significant changes in signal intensity were found almost exclusively in brain structures involved in olfactory processing. Most foci of signal intensity change were located in secondary olfactory areas, such as amygdala, entorhinal cortex, nucleus accumbens/septal nuclei, and some in orbital frontal cortex. Different sites of changes were found in different subjects, possibly due to the relatively low sensitivity of this novel brain imaging method. Pleasant and unpleasant odors generally activated different olfactory structures, possibly reflecting the difference in affective evaluation. In addition to functional MRI we are also using 15-0 PET scans during olfactory stimulation in order to compare the distribution of changes in cerebral blood flow measured during PET with the results obtained by fMRI.